Any recommendations on sim cards for the above countries for a month?
The Vodafone UK "Big Value Bundles" are "30-day bundles of data, plus unlimited minutes and texts." Currently, they have bundles starting at £10 with 21 GB data and Unlimited minutes and texts that will work in the UK and Republic of Ireland.
We used GiffGaff sim cards while in Scotland last summer, and I plan to use the same card this Spring in Ireland. I'd rather have an Irish phone number, because of the messing about with country codes, etc., but $10 is less than the Irish providers are offering, and the only calling I do is to restaurants/bars for bookings and inquiries about live music. We use a GPS dashboard device so we don't need data except when lost. Using our US carrier is $10/day and has the same country code issues.
We also had a good experience with giffgaff SIM cards last year in England. Easy to use with the giffgaff app, and you can’t beat the price.